Synthesis And Thermal Stability Of Nitride Red Phosphors For White LED Applications

LEDs have been considered as the next-generation lighting sources due to their low-energy consumption,long serving time,and high light efficiency. The most widely adopted White LEDs are combinations of InGaN devices with 460 nm blue lights and yellow and red emitting phosphors.At present, the nitride phosphors are the most promising new red phosphors for LED red phosphors.They have many advantages such as: high light efficiency, high thermal quenching temperature,good chemical stability, and a broad absorption band spanning from UV to the green region of the electromagnetic spectrum,but the photoluminescence thermal degradation is a limitation for Sr₂Si₅N₈:Eu²⁺ phosphors because of the high temperature in white LED when it is working. And the thermal quenching properties of nitride phosphors also need to be stuied in details. Besides, the high synthesis pressure for ?Sr/Ca?AlSiN₃:Eu2+ is also a big problem. In this paper, the above problems are studied, mainly to solve the following problems:Firstly, the method of preparation of nitride phosphor using Li₃N flux was studied. The effect of Li₃N flux on the nitride phosphor structure, morphology and luminescent properties are discussed. The preparation temperature of Sr₂Si₅N₈:Eu²⁺ phosphor decreased a lot by using flux,and the luminescence properties of phosphors increased greatly. The thermal degradation properties of nitride phosphors have been improved by this synthesis method. Li₃N flux can also be used for the preparation of SrAlSiN3 phosphor. Adding flux could prepare SrAlSiN3 phosphor under atmospheric pressure which usually needs high pressure conditions to react, and flux is benefit for improving the luminescent properties. Li₃N flux is also expected to be used for other nitride phosphor preparation to enhance its luminescent properties.Secondly, to study the possible thermal degradation mechanism, the effect factors for thermal degradation of nitride phosphors and matrix have been examined and discussed. Through the testing and analysis methods, the formation of an amorphous layer on the grain surface from the oxidation of Sr2Si5N8 host lattice caused the decreasing of luminescence effciency.To improve the thermal luminescence performance of nitride phosphors, some metrods including chemical bond substitution, coating, atmosphere protection heat treatment, and the flux preparation have been studied. The atmosphere protection heat treatment and preparation of nitride system with flux have good effect on improving the thermal degradation properties of nitride phosphors.Finally, the thermal quenching properties of nitride phosphors were also studied. The thermal quenching properties of the Eu2+ excited nitride phosphor were closely related to the activator content. With the increase of the activator content, the thermal quenching increased. In Sr2-xEuxSi5N8 system, because there are two different Eu2+ ions occupied positions, the emitting spectrum has a red shift due to the energy transfer. For ?Sr/Ca?AlSiN3:Eu2+ system, the thermal quenching of pure Sr or Ca sample is the lowest at same heating temperature,the degree of thermal quenching increased slightly with the increase of Ca content.